How to activate Windows XP by phone
To contact a Microsoft customer service representative to activate Windows by phone, follow these steps:

Click Start, point to All Programs, point to Accessories, point to System Tools, and then click Activate Windows.
Or, click the Windows Activation icon in the notification area.
Click Yes, I want to telephone a customer service representative to active Windows now.
Click Read the Windows Product Activation Privacy Statement, click Back, and then click Next.
Follow the steps in the Activate Windows by phone dialog box, and then click Next.
Note The number appears now and differs based on the location that you select.
When activation is completed and you receive the following message, click OK.

I added more RAM, upgraded the power supply and changed the CPU fan for a cooler version. But that was all at least 4 or 5 years ago, and the rest of the PC is as it was when bought. Haven't changed anything recently though.

Thanks for the guidance, I will give it a go. Still curious as to why it has suddenly done it though. A cynic might think it's to get me to buy Windows 7.....